How Does Sickle Cell Cause Disease?

The Mutation in Hemoglobin

Sickle
cell disease is a blood condition seen most commonly in people of African
ancestry and in the tribal peoples of India.
Clinically significant sickle cell syndromes also occur in people of Mediterranean
and Middle Eastern background. Here, the most common problem is a combination
sickle cell and beta thalassemia genes. The sickle cell mutation reflects
a single change in the amino acid building blocks of the oxygen-transport
protein, hemoglobin. This protein, which is the component that gives red
cells their color, has two subunits. The alpha subunit is normal in people
with sickle cell disease. The beta subunit has the amino acid valine at
position 6 instead of the glutamic acid that is normally present. The alteration
is the basis of all the problems that occur in people with sickle cell
disease. The schematic diagram shows the first eight of the 146 amino acids
in the beta globin subunit of the hemoglobin molecule. The amino acids
of the hemoglobin protein are represented as a series of linked, colored
boxes. The lavender box represents the normal glutamic acid at position
6. The dark green box represents the valine in sickle cell hemoglobin.
The other amino acids in sickle and normal hemoglobin are identical.

Figure 1.The chain of colored boxes represent the first eight amino acids in the beta chain of hemoglobin. The sixth position in the normal beta chain has glutamic acid, while sickle beta chain has valine. This is the sole difference between the two.

The molecule, DNA (deoxyribonucleic acid), is the fundamental
genetic material that determines the arrangement of the amino acid building
blocks in all proteins. Segments of DNA that code for particular proteins
are called genes. The gene that controls the production of the beta globin
subunit of hemoglobin is located on one of the 46 human chromosomes (chromosome
#11). People have twenty-two identical chromosome pairs (the twenty-third
pair is the unlike X and Y chromosomes that determine a person's sex).
One of each pair is inherited from the father, and one from the mother.
Occasionally, a gene is altered in the exchange between parent and offspring.
This event, called mutation, occurs extremely rarely. Therefore, the inheritance
of sickle cell disease depends totally on the genes of the parents.

If only one of the beta globin genes is the "sickle" gene and
the other is normal, the person is a carrier for sickle cell disease. The
condition is called sickle
cell trait. With a few rare exceptions, people with sickle cell trait
are completely normal. If both beta globin genes code for the sickle protein,
the person has sickle cell disease. Sickle cell disease is determined at
conception, when a person acquires his/her genes from the parents. Sickle
cell disease cannot be caught, acquired, or otherwise transmitted.
Also, sickle cell trait does not develop into sickle cell disease. Sickle cell trait
partially protects people from the deadly consequences of malaria. The frequency
of the sickle cell gene reached high levels in Africa and India due to the protection
against malaria that occurred for people with sickle cell trait.

The hemoglobin molecule (made of alpha and beta globin subunits)
picks up oxygen in the lungs and releases it when the red cells reach peripheral
tissues, such as the muscles. Ordinarily, the hemoglobin molecules exist
as single, isolated units in the red cell, whether they have oxygen bound
or not. Normal red cells maintain a basic disc shape, whether they are
transporting oxygen or not.

The
picture is different with sickle hemoglobin (Figure 2). Sickle hemoglobin exists as
isolated units in the red cells when they have oxygen bound. When sickle
hemoglobin releases oxygen in the peripheral tissues, however, the molecules
tend to stick together and form long chains or polymers. These rigid polymers
distort the cell and cause it to bend out of shape. While most distorted
cells are simply shaped irregularly, a few have a cresent-like appearence
under the microscope. These cresent-like or "sickle shaped" red cells gave
the disorder its name. When the red cells return to the lungs and pick
up oxygen again, the hemoglobin molecules resume their solitary existence
(the left of the diagram).

A single red cell may traverse the circulation four times
in one minute. Sickle hemoglobin undergoes repeated episodes of polymerization
and depolymerization. This cyclic alteration in the state of the molecules
damages the hemoglobin and ultimately the red cell itself.

Polymerized sickle hemoglobin does not form single strands. Instead,
the molecules group in long bundles of 14 strands each that twist in a
regular fashion, much like a braid (Figure 3).

Schematic Representaion of Polymerized Sickle Hemoglobin

Figure 3.Polymers of deoxygenated sickle hemoglobin molecules. Each hemoglobin
molecule is represented as a sphere. The spheres twist in an alpha helical bundle
made of 14 sickle hemoglobin chains.

These bundles self-associate into even larger structures that stretch and
distort the cell. An analogy would be a water balloon which was stretched
and deformed by icicles. The stretching of the balloon's rubber is
similar to what happens to the membrane of the red cell. Polymers tend
to grow from a single start site (called a nucleation site) and often grow
in multiple directions. Star-shaped clusters of hemoglobin S polmers develop
commonly.

Despite their imposing appearance, the sickle hemoglobin polymers
are held together by very weak forces. The abnormal valine amino acid at
position 6 in the beta globin chain interacts weakly with the beta globin
chain in an adjacent sickle hemoglobin molecule. The complex twisting,
14-strand structure of the bundles produces multiple interactions and cross-interactions
between molecules. The weak nature of the interaction opens one strategy
to treat sickle cell disease.

Some types of hemoglobin molecules, such as that found
before birth (fetal hemoglobin),
block the interactions between the deoxygenated hemoglobin S molecules.
All people have fetal hemoglobin in their circulation before birth. Fetal
hemoglobin protects the unborn child and newborns from the effects of sickle
cell hemoglobin. Unfortunately, this hemoglobin disappears within the first
year after birth. One approach to treating sickle cell disease is to rekindle
production of fetal hemoglobin. The drug, hydroxyurea
induces fetal hemoglobin production in some patients with sickle cell disease
and improves the clinical condition
of some people.

The Sickle Red Cell

Capillary Flow of Normal and Sickle Red Cells

Figure 4.Normal red cells maintain their shape as they pass through the capillaries
and release oxygen to the peripheral tissues (upper panel). Hemoglobin polymers form
in the sickle rell cells with oxygen release, causing them to deform. The deformed
cells block the flow of cells and interrupt the delivery of oxygen to the tissues
(lower panel).

Figure 4 shows the changes that occur as sickle or normal red
cells release oxygen in the microcirculation. The upper panel shows that
normal red cells retain their biconcave shape and move through the smallest
vessels (capillaries) without problem. In contrast, the hemoglobin polymerizes
in sickle red cells when they release oxygen, as shown in the lower panel.
The polymerization of hemoglobin deforms the red cells. The problem, however,
is not simply one of abnormal shape. The membranes of the cells are rigid
due in part to repeated episodes of hemoglobin polymerization/depolymerization
as the cells pick up and release oxygen in the circulation. These rigid
cells fail to move through the small blood vessels, blocking local blood
flow to a microscopic region of tissue. Amplified many times, these episodes
produce tissue hypoxia (low oxygen supply). The result is pain, and often
damage to organs.

The damage to red cell membranes promotes many of the complications
of sickle cell disease. Robert Hebbel at the University of Minnesota and
colleagues were among the first workers to show that the heme
component of hemoglobin tends to be released from the protein with repeated
episodes of sickle hemoglobin polymerization. Some of this free heme lodges
in the red cell membrane. The iron in the center of the heme molecule promotes
formation of very dangerous compounds, called reactive oxygen species.
These molecules damage both the lipid and protein components of the red
cell membrane. Membrane stiffness is one of the consequences of this injury.
Also, the damaged proteins tend to clump together to form abnormal clusters
in the red cell membrane. Antibodies develop to these protein clusters,
leading to even more red cell destruction (hemolysis).

The anemia in sickle cell disease is caused by red cell destruction,
or hemolysis. The production of red cells by the bone marrow increases
dramatically, but is unable to keep pace with the destruction. Red cell
production increases by five to ten-fold in most patients with sickle cell
disease. The average half-life of normal red cells is about 40 days. In
patients with sickle cell disease, this value can fall to as low as four
days. The volume of "active" bone marrow is much greater than normal in
patients with sickle cell disease due to the demand for greater red cell
production.

The degree of anemia varies widely between patients. In general,
patients with sickle cell disease have hematocrits that are roughly half
the normal value (e.g., about 25% compared to about 40-45% normally).
Patients with hemoglobin SC disease (where one of the beta globin genes
codes for hemoglobin S and the other for the variant, hemoglobin C) have
higher hematocrits than do those with homozygous Hb SS disease. The hematocrits
of patients with Hb SC disease run in low- to mid-thirties. The hematocrit
is normal for people with sickle cell trait.